CHAPTER 9 A VIEW OF EARTH’S PAST
I. GEOLOGIC TIME
- geologic time scale – a scale that outlines the development of Earth and
of life on Earth
A. Geologic Column
1. 19th century scientists determined the relative ages of sedimentary
rock in different areas around the world
a. studying fossils
b. applying Law of Superposition
2. no single area on Earth contained a record of all geologic time
3. scientists developed the geologic column
a. an ordered arrangement of rock layers that is based on the
relative ages of the rocks and in which the oldest rocks are at
bottom
b. rock layers are distinguished by the types of rocks
c. rock layers are distinguished by the kinds of fossils located
1c. fossils in the upper rock layers resemble modern plants
and animals
2c. fossils in lower rock layers are of plants and animals that
are different from those living today
3c. many fossils discovered in old layers are from species that
are extinct
4. Using a Geologic Column
a. first columns developed estimated age or rocks by rate of
deposition
b. radiometric dating allowed scientists to determine absolute age
more accurately
c. scientists use more geologic columns to determine absolute age
without radiometric dating by using index fossils
B. Divisions of Geologic Time
1. the geologic history of Earth is marked by major changes
a. Earth’s surface features
b. climate changes
c. types of organisms
2. geologists use the above indicators to divide the geologic time scale
into smaller units
3. units of geologic time is generally characterized by fossils of a
dominant life form (Table 1, page 213)
4. the abbreviation “Ma” stands for mega-annum
a. means “one million years”
b. used to better discuss geologic time
5. Eons and Eras
a. eon is the largest unit of geologic time
b. geologic time is divided into four eons
1b. Hadean Eon
2b. Archean Eon
3b. Proterozoic Eon
4b. Phanerozoic Eon
c. Precambrian Time
1c. contains the first three eons
2c. four billion years
3c. very few fossils (why?)
aa. organisms were very primitive
bb. much of the rock from that time is either igneous
or metamorphic
d. eras
1d. a unit of geologic time that includes two or more periods
2d. smaller than an eon
3d. Phanerozoic Eon
aa. Paleozoic Era
1aa. first era of Phanerozoic Eon
2aa. lasted 291 million years
3aa. fossils were a wide variety of marine and
terrestrial life forms
bb. Mesozoic Era
1bb. lasted 186 million years
2bb. fossils include early forms of birds and reptiles
cc. Cenozoic Era
1cc. present age
2cc. began 65 million years ago
3cc. fossils of mammals are common
6. Periods and Epochs
a. period
1a. a unit of geologic time that is longer than an epoch but
shorter than an era
2a. characterized by specific fossils
3a. usually named for the location in which the fossils were
first discovered
b. epoch
1b. a subdivision of time that is longer than an age but shorter
than a period
2b. created by certain criteria
aa. rock record is most complete
bb. rock record is least deformed
cc. detailed fossil record
3b. ages
aa. subdivision of epoch
bb. defined by the occurrence of distinct fossils